1. Field of the Invention
The present invention relates to a color measuring device, an image forming apparatus, a color measuring method, and a color measuring system.
2. Description of the Related Art
An image forming apparatus, such as a color inkjet image forming apparatus or a color electrophotographic image forming apparatus, is becoming able to provide a higher image quality and is increasingly used for offset printing of advertising media, brochures, or the like, for which high-quality images are required although the number of copies to be printed is relatively small.
In the offset printing that requires a high image quality, in some cases, colors actually printed out by the image forming apparatus may differ from colors of a printing material desired by a customer.
In general, the customer checks the colors of the printing material on a display and then orders printing. However, the image forming apparatus has specific color reproduction characteristics according to each model, so that printed colors may differ from the colors checked on the display.
Therefore, a technology has been employed to reproduce colors by using a color space, such as the Lab color space or the xyz color space, that is independent of a device, such as a display or an image forming apparatus.
In addition, the image forming apparatus controls the amount of color materials in order to output a designated color. For example, an inkjet image forming apparatus calculates an ink ejection amount or a print pattern to control the amount of ink ejected from an ink head, thereby controlling output colors. For another example, an electrophotographic image forming apparatus controls the amount of toner attached to a photoreceptor or controls the intensity of a laser beam, thereby controlling output colors.
However, the amount of color materials, such as the ink ejection amount of the inkjet image forming apparatus, varies depending on the state of a head nozzle, depending on a variation in the viscosity of ink, or depending on a variation in an ejection driving element (e.g., a piezoelectric element), resulting in a variation in the color reproducibility. Moreover, the ink ejection amount of the inkjet image forming apparatus may change over time in the same image forming apparatus or may vary between different image forming apparatuses. Therefore, the color reproducibility of an image varies over time or between image forming apparatuses.
Therefore, a technology has been employed, in which a test pattern is printed out, colors of the printed test pattern are measured, and the ink ejection amount or image data is corrected so that a target color can be reproduced in a printed material.
Specifically, there has been proposed a method for generating a color conversion table to be referred to for converting a primary color value in a primary color space into a target color value in a target color space (see Japanese Patent Application Laid-open No. 2006-222783). In the conventional technology, a plurality of color patches contained in a predetermined region near a lattice-point of interest in the primary color space are selected from among color patches, primary color values in the primary color space are obtained by capturing the selected color patches by a photoelectric conversion element, intermediate color values in an intermediate color space are obtained by measuring colors of the selected color patches, and an individual transformation matrix coefficient to transform the primary color space into the intermediate color space is obtained for the lattice-point of interest by using a set of the primary color values and the intermediate color values of the selected color patches. Subsequently, the above process is repeated to obtain the individual transformation matrix coefficients of a plurality of lattice-points of interest, and thereafter, individual intermediate color values corresponding to the respective lattice-points of interest are calculated by using the obtained individual transformation matrix coefficients, target color values in the target color space corresponding to the calculated individual intermediate color values are obtained, and the lattice-points of interest and the target color values are associated with each other, so that the color conversion table for transforming the primary color space into the target color space is generated.
More specifically, in the conventional technology, a color transformation matrix is generated based on the intermediate color values of the color patches whose colors are measured in only a local space of the lattice-point of interest by a spectrophotometric color measuring device, and then a table for transforming the primary color space into the target color space is generated.
However, in the conventional technology, the color transformation matrix is generated based on the intermediate color values of the color patches whose colors are measured in only the local space of the lattice-point of interest by the spectrophotometric color measuring device and then the table for transforming the primary color space into the target color space is generated; therefore, to correct the color reproducibility influenced by a temporal change or by an individual variability of an image forming device, such as a recording head of an inkjet image forming apparatus or an image forming unit of an electrophotographic image forming apparatus, it is necessary to frequently update the color transformation matrix by measuring colors by the spectrophotometric color measuring device. Therefore, costs for the image forming apparatus itself or operation costs increase. Furthermore, in the conventional technology, because the color transformation matrix used to generate the conversion table to transform the primary color space into the target color space is generated by using the intermediate color values of the color patches whose colors are measured in only the local space of the lattice-point of interest, the local space region is separated. Therefore, if sample data points are disproportionately distributed in a selected neighboring space, a distortion error in a linear transformation result increases, resulting in the reduced color reproducibility.
Therefore, there is a need to realize stable color reproduction with reduced distortion at low costs.